The ring-opening polymerization of bimolecular cyclic esters of hydroxycarboxylic acid (also called “cyclic dimmers”) may yield polyhydroxycarboxylic acids. Typical of such cyclic esters are glycolide that is a bimolecular cyclic ester of glycolic acid and lactide that is a bimolecular cyclic ester of lactic acid. The ring-opening polymerization of glycolide yields polyglycolic acid (i.e., polyglycolide), and the ring-opening polymerization of lactide yields polylactic acid (i.e., polylactide).
Polyglycolic acid and polylactic acid obtained by the ring-opening polymerization of cyclic esters or polyhydroxycarboxylic acids such as ring-opened copolymers of lactide and glycolide have been known as biodegradable polymer materials, and their application to surgical sutures, etc. have been long proposed (for instance, U.S. Pat. Nos. 3,297,033 and 3,636,956).
Polyglycolic acid in particular, because of being better in heat resistance, gas barrier properties, mechanical strength, etc. than other biodegradable polymer materials, is finding new applications to sheets, films, vessels, injection-molded articles and so on (Japanese Patent Application Laid-open (A) Nos. 10-60136, 10-80990, 10-138371 and 10-337772).
These polyhydroxycarboxylic acids have difficulty in controlling their rate of biodegradation, although they are biodegradable and environmentally friendly polymer materials. So far, the rate of biodegradation of polyhydroxycarboxylic acids has generally been thought of as being dependent on their average molecular weight. The rate of biodegradation may be quantitatively determined to a certain degree by burying, for instance, a polyhydroxycarboxylic acid molded article in the ground to observe the period of its disintegration. This method is called soil degradability test.
When polyhydroxycarboxylic acid molded articles are tested for their degradability in the ground, it has so far been considered that the higher the weight-average molecular weight of polyhydroxycarboxylic acids, the longer the period of time needed for disintegration becomes, and the lower the weight-average molecular weight, the shorter the period of disintegration time becomes. It is understood that when polyhydroxycarboxylic acids have a very low weight-average molecular weight, their time of disintegration in the ground is generally short.
However, the results of the inventors' studies have indicated that the rate of biodegradation of polyhydroxycarboxylic acids is not necessarily dependent on their average molecular weight such as weight-average molecular weight. The same holds true even when instead of weight-average molecular weight, solution viscosity, melt viscosity and so on are used as the index to average molecular weight.
In general, when polyhydroxycarboxylic acids have a fast rate of biodegradation, they have some merits: biodegradation of used-up polyhydroxycarboxylic acid molded articles and ease with which they can be composted. However, such molded articles have limited applications to very-short-time fields or low-strength fields.
When polyhydroxycarboxylic acid molded articles such as films or containers are used in application fields where durability and outside shape retention on much the same order as in ordinary plastic molded articles are expected, too a fast rate of biodegradation causes premature drops of articles' strength, and makes it difficult to retain the outside shape of articles over an extended period of time. Thus, there have been attempts to obtain molded articles improved in durability and outside shape retention without detriment to their biodegradability by allowing polyhydroxycarboxylic acids to have higher molecular weight.
Contrary to expectation, however, it has been found that only by use of high-molecular-weight polyhydroxy-carboxylic acids, it is still difficult to keep hold of strength and outside shape while premature biodegradability is fully minimized. In addition, it is still difficult to make products of consistent quality because there are variations in the rate of biodegradation for each polyhydroxycarboxylic acid production lot. On the other hand, polyglycolic acids obtained by the ring-opening polymerization of glycolide are vulnerable to coloration upon polymerization at elevated polymerization temperatures for long periods of time.
Thus, it is still difficult to control the rate of biodegradation of polyhydroxycarboxylic acids while their coloration is reduced, and anything significant about how to achieve this is not proposed at all.
Referring to the polyglycolic acid encompassed in polyhydroxycarboxylic acids, there is unavailable any well-established production technique as yet, and so it is still difficult to produce polyglycolic acid that can yield less colored molded articles.
Polyglycolic acid, when it is poor in melt stability, makes it impossible to mold its melt in a stable manner. Polyglycolic acid, when it is vulnerable to coloration, detracts from commercial value, and offers hygienic problems as well. When polyglycolic acid has a fast rate of biodegradation, it is difficult to put the service life of product under control although the polyglycolic acid can be easily composted.
U.S. Pat. No. 3,297,033 discloses that ring-opening polymerization is carried out at 185 to 190° C. while glycolide mixed with a polymerization catalyst is charged into a glass tube, and that white polymers are obtained after cooling (Example 1). By carrying out the ring-opening polymerization at temperatures lower than the melting point (about 220° C.) of polyglycolic acid, it is thus possible to obtain less colored polymers.
However, lower polymerization temperatures render the resulting polymer likely to crystallize and solidify during polymerization reactions, whereby the polymerization reactions tend to become inhomogeneous. The resulting polyglycolic acid is so poor in melt stability that when extrusion molded into various articles such as sheets, films and fibers, it is difficult to carry out extrusion molding in a stable fashion because of large melt viscosity changes.
U.S. Pat. No. 3,468,853 discloses a process wherein glycolide mixed with a polymerization catalyst is subjected to ring-opening polymerization at a temperature of 205 to 235° C. until viscosity reaches a substantial equilibrium. However, long-term ring-opening polymerization at elevated temperatures often causes the resulting polyglycolic acid to be colored, greatly detracting from commercial value.
U.S. Pat. No. 2,668,162 discloses a polyglycolic acid production process wherein glycolide mixed with a polymerization catalyst is subjected to ring-opening polymerization at 150 to 200° C. to produce a low-molecular-weight polymer, and the polymer is then heated to 220 to 245° C. to increase its melt viscosity. With this process, however, it is difficult to prevention coloration of the resulting polyglycolic acid because a time-consuming heating step is needed and rapid heating tends to lead to heating variations.